1. Field of the Invention
The present invention relates to detecting angular acceleration (rotational acceleration) and translational acceleration resulting from shock to an electronic device.
2. Description of Related Art
Miniaturization of electronic components has helped drive the rapid adoption and distribution of notebook computers and other types of portable electronics. This has also increased demand for small, surface-mountable, high performance acceleration (shock) detectors in order to assure and improve the reliability of these electronic devices. This is because physical shocks to a high density magnetic storage device while writing to the storage medium can, for example, cause the position of the write head to shift. This can result in data write errors and corruption or even damage to the head. It is therefore necessary and desirable to detect shock to the magnetic storage device and either interrupt the write operation or move the head to a safe position.
As the recording density of magnetic storage devices has increased, the track width on the disk surface has narrowed. This makes it even easier for the position of the head to shift (for the track to shift) at the slightest vibration. Another problem is that, in addition to impact and vibration from external sources acting on the magnetic storage device, small vibrations from the spinning of motors inside the magnetic storage device can also cause the magnetic head to skip tracks.
Vibrations acting on the magnetic storage device include both translational vibration and rotational vibration. For control purposes, it is therefore necessary to distinguish translational acceleration from angular acceleration (referred to below as rotational acceleration), and a sensor capable of detecting translational acceleration and rotational acceleration is needed. Translational acceleration can be detected using a single prior art acceleration sensor. Rotational acceleration can be detected with the greatest sensitivity using two or more acceleration sensors placed as far from each other and from the axis of rotation as possible. If two acceleration sensors are placed equidistantly from and on opposite sides of the rotational axis, the output signals from the acceleration sensors will be opposite sign but the same magnitude when rotational acceleration occurs. This will, however, only be true when the center of rotation is centered between the two acceleration sensors. Furthermore, when two acceleration sensors having the same practical characteristics are located on the same side of relative to the axis of rotation, translational acceleration will be the same at each acceleration sensor and the output signals from the acceleration sensors in response to translational acceleration will have the same magnitude. On the other hand, when rotational acceleration occurs, the output signals from the two acceleration sensors will differ in magnitude because the distance from the axis of rotation to each acceleration sensor differs. Rotational acceleration can therefore be detected by obtaining the difference between the two output signals.
A piezoelectric element, which produces a voltage as a result of deformation of a piezoelectric body in response to strain, can also be used as an acceleration sensor as described in Japanese Patent Laid-open Publication (kokai) No. 10-96742. One piezoelectric element used for an acceleration sensor is flat with a flat cantilevered portion. Strain deformation from acceleration is picked up as vibration of the cantilever portion of the piezoelectric body, thus producing a charge that can be detected to detect acceleration.
The problem is that when the acceleration sensor consists of plural piezoelectric elements, differences occur in the characteristics of the individual piezoelectric elements.
Furthermore, when the acceleration sensor has two piezoelectric elements disposed at separate positions, the sensitivity of the piezoelectric elements may be affected by where the elements are positioned. For example, temperature differences resulting from the position of the elements can offset the sensitivity of each piezoelectric element. In this case differences in the output signals from each piezoelectric element can cause rotational acceleration to be mistakenly detected when translational acceleration occurred, and rotational acceleration cannot be accurately recognized.
Yet further, reducing the distance between piezoelectric elements by housing two piezoelectric elements in the confined space inside a single package also reduces the detection signal difference. As a result, rotational acceleration cannot be detected with high sensitivity.
With consideration for the problems described above, it is therefore an object of the present invention to provide an acceleration sensor that can be disposed inside a limited space and can detect rotational acceleration with high sensitivity.
To achieve this object, an acceleration sensor according to the present invention has first piezoelectric element having electrodes for outputting a charge produced by strain deformation and second piezoelectric element having electrodes for outputting a charge produced by strain deformation. The first piezoelectric element has at least one piezoelectric body and a support block supporting the piezoelectric body. The second piezoelectric element has at least one piezoelectric body and a support block supporting the piezoelectric body. The electrodes are provided on opposite surfaces of the piezoelectric element, and one surface of the first piezoelectric element and one surface of the second piezoelectric element are effectively parallel to each other.
One surface of the first piezoelectric element and one surface of the second piezoelectric element are substantially parallel to each other so that the vibrating surfaces of the piezoelectric elements are parallel and each piezoelectric element will thus vibrate in the same direction in response to acceleration in a single direction. Yet further preferably, one surface of the first piezoelectric element and one surface of the second piezoelectric element are in the same plane. The piezoelectric elements normally vibrate perpendicular to the cantilever surface, and the surfaces of the cantilever portions of the piezoelectric elements are therefore preferably parallel to each other.
Yet further preferably, the cantilever portion and support block portion of the piezoelectric body in each piezoelectric element are a continuous monolithic portion, but the support block portion can be separate from the cantilever portion.
This configuration makes it possible to dispose first and second piezoelectric elements for detecting acceleration in the same direction in a confined space, and detect rotational acceleration with high sensitivity due to the distance between the support blocks supporting the piezoelectric body of the piezoelectric elements. Rotational acceleration can also be detected without being affected by environmental factors due to the installation.
Further preferably, the first and second piezoelectric elements are cantilevered, having a cantilever portion including a main piezoelectric body surface and a support block portion for supporting the piezoelectric body. The first and second piezoelectric elements are aligned with the longitudinal axes of the cantilever portion with the support block portions disposed to the outside mutually distant in opposite directions along the longitudinal axis, a free end portion of the cantilever of the first piezoelectric element and a free end portion of the cantilever of the second piezoelectric element are pointing to each other. Each free end portion of cantilever is located in mutual proximity to the inside, and the ends of the cantilever portions are substantially mutually parallel.
Yet further preferably, the first piezoelectric element includes one piezoelectric body and the second piezoelectric element includes one piezoelectric body. The direction of polarization of the piezoelectric body of the first piezoelectric element and the direction of polarization of the piezoelectric body of the first piezoelectric element are opposite. It is noted that the direction of polarization may be called the polarized direction.
Alternatively, the first piezoelectric element includes one piezoelectric body and the second piezoelectric element includes one piezoelectric body. The direction of polarization (polarized direction) of the piezoelectric body of the first piezoelectric element and the direction of polarization of the piezoelectric body of the second piezoelectric element are the same.
Alternatively, the first piezoelectric element includes a plurality of layers of bonded piezoelectric bodies and the second piezoelectric element includes a plurality of layers of bonded piezoelectric bodies. Thus bonding plural piezoelectric bodies to form the piezoelectric elements produces a stronger output signal.
Yet further preferably, the direction of polarization of all piezoelectric bodies forming the piezoelectric elements is the same in each of the first and second piezoelectric elements.
Alternatively, the directions of polarization of the piezoelectric bodies constituting the first piezoelectric element and the directions of polarization of the piezoelectric bodies constituting the second piezoelectric element are mutually opposite.
Yet further preferably, the direction of polarization of the piezoelectric bodies constituting the first piezoelectric element and the direction of polarization of the piezoelectric bodies constituting the second piezoelectric element are the same.
Yet further preferably, the first piezoelectric element includes at least two piezoelectric bodies and the second piezoelectric element includes at least two piezoelectric bodies. The piezoelectric bodies of each piezoelectric element are bonded with surfaces of the same polarity of polarization. Therefore, the direction of one piezoelectric body and the direction of another piezoelectric body are opposite.
Yet further preferably, the directions of polarization of the corresponding piezoelectric bodies are mutually opposite in the first and second piezoelectric elements.
Alternatively, the directions of polarization of the corresponding piezoelectric bodies are the same in the first and second piezoelectric elements.
Yet further preferably, in one piezoelectric element the piezoelectric bodies of the piezoelectric element are bonded with an intervening shim therebetween.
Substantially any material that can bond with the piezoelectric body can be used for the shim. Preferably, however, the shim will pass vibration due to acceleration to the piezoelectric body. Yet further preferably the shim is a silicon substrate.
Yet further preferably, the piezoelectric element is formed by bonding the plural piezoelectric bodies by direct bonding.
An adhesive layer is thus not formed at the interface between the piezoelectric bodies. Vibration due to acceleration is thus not absorbed by an adhesive layer, and acceleration can be detected with high sensitivity due to device stability.
Yet further preferably, the piezoelectric element is formed by bonding a plurality of piezoelectric bodies by direct bonding by way of at least one of oxygen atoms and hydroxyl groups.
The piezoelectric bodies can thus be strongly bonded to each other as a result of the oxygen atoms or hydroxyl groups.
Yet further preferably, the acceleration sensor has an output terminal for each electrode of the first and second piezoelectric elements.
Yet further preferably, the acceleration sensor also has for each of the first and second piezoelectric elements at least one output terminal interconnecting electrodes of mutually different generated charge polarity between different piezoelectric elements.
By connecting an electrode of one charge polarity in the first piezoelectric element to an electrode of the opposite charge polarity in the second piezoelectric element, and connecting the node therebetween as the output terminal, the charges from the two piezoelectric elements are mutually cancelling, and the excess charge is obtained as the difference between the piezoelectric element outputs. When rotational acceleration occurs the charges generated by the piezoelectric elements differ according to the distance from the center of rotation. On the other hand, the charges generated by the piezoelectric elements due to translational acceleration are the same. It is therefore possible to detect rotational acceleration based on the difference between the outputs. In other words, interconnecting the piezoelectric elements as described above obtains the difference of the outputs. It is therefore not necessary to provide an external differential circuit.
Yet further preferably, electrodes of the same generated charge polarity in different piezoelectric elements are connected, and the first and second piezoelectric elements each comprise an output terminal from an electrode other than the connected electrodes.
In this case electrodes of the same generated charge polarity are connected in series between the first and second piezoelectric elements, charges of the same polarity are mutually cancelling, and the difference between the charges generated by the piezoelectric elements is output. Any excess charge can thus be obtained as the difference in piezoelectric element output, and rotational acceleration can be detected from this difference. In other words, interconnecting the piezoelectric elements as described above obtains the difference of the outputs. It is therefore not necessary to provide an external differential circuit.
Yet further preferably, the acceleration sensor also has at least one output terminal set for outputting a charge generated at each electrode of the first and second piezoelectric elements.
Yet further preferably, the first piezoelectric element is effectively adjusted to the same sensitivity as the second piezoelectric element.
The first and second piezoelectric elements of this acceleration sensor normally have substantially the same sensitivity by manufacturing both piezoelectric elements to the same dimensions. However, to further improve acceleration detection sensitivity, the sensitivity of one piezoelectric element is preferably adjusted to effectively the same sensitivity as the other piezoelectric element. Adjusting piezoelectric element sensitivity can be accomplished by, for example, removing portion of the cantilever portion or adding a sensitivity adjusting mass to the cantilever portion.
Therefore, a portion of the cantilever portion of the first piezoelectric element is preferably removed in another acceleration sensor of this invention.
Alternatively, a sensitivity adjusting mass is affixed the cantilever portion of the first piezoelectric element in another acceleration sensor of this invention.
Yet further preferably, the first piezoelectric element is fixed by the support block portion inside a package and second piezoelectric element is fixed by the support block portion inside a package so that the cantilever portion can vibrate freely.
By thus housing the first and second piezoelectric elements in a package, output can be easily obtained from the electrodes of the piezoelectric elements.
Yet further preferably, the first and second piezoelectric elements are mounted inside the package with the cantilever portion inclined to the surface of the package.
By thus mounting the piezoelectric elements with the cantilever inclined to the surface of the package, the cantilever will also vibrate at an incline to the surface. It is therefore possible to detect acceleration parallel to the package surface as well as acceleration perpendicular to the package surface.
Yet further preferably, the first and second piezoelectric elements are mounted to the package so that the angle of inclination between the cantilever portions thereof and the package surface is mutually different.
Yet further preferably, two sets of piezoelectric elements are mounted in the package, the first and second piezoelectric elements of the first set mounted with the cantilever portion thereof perpendicular to the package surface, and the first and second piezoelectric elements of the second set mounted with the cantilever portion parallel to the package surface.
By thus providing two sets of piezoelectric elements mounted with the cantilever portions thereof respectively parallel and perpendicular to the package surface, acceleration components parallel and perpendicular to the package surface can be separately detected. It will also be obvious that a third set of piezoelectric elements could be added to detect acceleration components in a third axial direction.
This invention also provides an acceleration detection apparatus having an acceleration sensor according to the present invention and a signal processing circuit for processing output signals from the piezoelectric elements.
It is thus possible to contain both the acceleration sensor and a semiconductor element integrating the signal processing circuit in a single package, thereby shortening the wiring, making the acceleration detection apparatus more resistant to noise, and able to detect acceleration with high sensitivity.
Yet further preferably, the first and second piezoelectric elements are connected to output to the signal processing circuit same-polarity output signals for acceleration in the same direction; and the signal processing circuit determines the difference between the output signals.
Alternatively, the first and second piezoelectric elements are connected to output to the signal processing circuit opposite-polarity output signals for acceleration in the same direction; and the signal processing circuit determines the sum of the output signals.
Yet further preferably, the signal processing circuit comprises a circuit for detecting angular acceleration from the difference of the outputs from the first and second piezoelectric elements.
Rotational acceleration acting on first and second piezoelectric elements at different distances from the center of rotation will be different. Translational acceleration, however, will be the same. Therefore, by detecting the difference of the output from the two piezoelectric elements, output signal components relating to translational acceleration will cancel, and the output signal due to rotational acceleration can be detected.
Yet further preferably, the signal processing circuit adjusts output so that the sensitivity of the first and second piezoelectric elements is effectively equal.
Yet further preferably, the signal processing circuit comprises one impedance converting circuit for converting output signal impedance from the piezoelectric elements, and an amplifier circuit for amplifying the converted output signals.
Yet further preferably, the signal processing circuit comprises two impedance converting circuits for converting output signal impedance from the piezoelectric elements, and an adding circuit for adding the converted output signals.
Yet further preferably, the signal processing circuit comprises two impedance converting circuits for converting output signal impedance from the piezoelectric elements, and a differential amplifier circuit for detecting and amplifying the converted output signal difference.
Yet further preferably, the acceleration detection apparatus has a plurality of output terminals for simultaneously externally outputting the amplified output of the converted output after impedance conversion, and the impedance-converted output signal of at least one piezoelectric element.
Yet further preferably, the first and second piezoelectric elements are fixed at the support block portion in the package so that the cantilever portions can vibrate freely, and the signal processing circuit is also housed inside the package.
A positioning apparatus according to the present invention has an acceleration detection apparatus according to the present invention for detecting acceleration, a moving means for moving an object, and a control means for controlling the moving means. The control means controls the moving means to move and position the object based on an output signal from the acceleration detection apparatus corresponding to detected acceleration.
The object can therefore be accurately positioned even if external interference applies acceleration to the positioning apparatus.
Yet further preferably, the cantilever portions of the first and second piezoelectric elements of the acceleration detection apparatus are disposed effectively parallel to a means for supporting the object.
A disk recording and reading apparatus according to the present invention has an acceleration detection apparatus for detecting acceleration according to the present invention, a head moving means for moving a head for recording to and reading from the disk, and a control means for controlling the head moving means. The control means calculates movement of the head based on an output signal from the acceleration detection apparatus corresponding to detected acceleration, and moves and positions the head by means of the head moving means.
The head can therefore be accurately positioned even if external interference applies acceleration to the disk drive. As a result, the durability of the disk recording/reading mechanism can be improved, and a high density recording/reading apparatus can be achieved.
Yet further preferably, the cantilever portion of the first and second piezoelectric elements of the acceleration detection apparatus are disposed effectively parallel to an arm supporting the head.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.